The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While many varied packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted and interconnected on a small footprint substrate. The substrate may in general include a rigid, dielectric base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for connection of the die to a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound which provides a protective package.
A cross-sectional side view of a conventional semiconductor package 20 is shown in FIG. 1. Typical packages include a plurality of semiconductor die, such as flash memory die 22 and a controller die 24, supported on a substrate 26. Substrate 26 includes vias 30, electrical traces 32 and contact pads 34 for transferring signals between the semiconductor die 22, 24 and a host device in which the package is located. A solder mask layer 36 (omitted from FIG. 2 for clarity) may be applied to the top and bottom surfaces of the substrate 26. Die bond pads (not shown) may be formed on a surface of the semiconductor die 22, 24 to electrically couple the semiconductor die to the substrate by affixing wire bonds 33 between respective die bond pads and contact pads 34. Once all electrical connections are made, the die and wire bonds may be encapsulated in a molding compound 38 to seal the package and protect the die and wire bonds.
Although the controller die 24 is typically smaller than the memory die 22, it is known to mount the controller die directly to the substrate, for example given the large number of electrical connections between the controller die and the substrate. In such a configuration, the memory die 22 need to be supported above the substrate 26 and controller die 24. One known technique is to apply solder balls 40 on certain substrate contact pads 30, exposed through the solder mask layer 36, and then support the memory die 22 on the solder balls.
One drawback to such a technique is that the surface of the substrate 26 is taken up with vias 30, traces 32 and contact pads 34, all used for electrical connections to and from the substrate. FIG. 2 is a top view of a substrate 26 including a configuration of vias 30, traces 32, contact pads 34 and solder balls 40. A large number of contact pads 34 have wire bond connections (not shown in FIG. 2), and therefore cannot take solder balls 40. The vias 30 and traces 32 also take up space which could otherwise be used for solder balls. Consequently, not enough solder balls are provided on the substrate, and stress points develop in the memory die 22, for example during the high pressure molding compound encapsulation process. These stress points can crack or otherwise damage the memory die 22, especially given the extremely thin profile of current memory die.